The present invention relates to an aluminum alloy heat exchanger, and a method of producing a refrigerant tube used for the heat exchanger.
An aluminum alloy that is lightweight and exhibits excellent thermal conductivity has been normally used for automotive heat exchangers (e.g., evaporator or condenser). Such a heat exchanger has been normally produced by applying a fluoride flux to the surface of an aluminum alloy extruded tube (i.e., refrigerant tube), assembling a member (e.g., fin material) on the aluminum alloy extruded tube to form a given structure, and brazing the materials in a heating furnace under an inert gas atmosphere, for example.
A refrigerant tube of automotive heat exchangers is normally formed using an aluminum multi-port extruded tube that has a plurality of hollow portions that are defined by a plurality of partition walls. In recent years, since a reduction in weight of heat exchangers has been desired to reduce the fuel consumption of automobiles from the viewpoint of reducing environmental impact, a refrigerant tube has been reduced in thickness. Therefore, the cross-sectional area of the refrigerant tube has further decreased, and a several hundred to several thousand extrusion ratio (cross-sectional area of container/cross-sectional area of extruded product) has been employed. Therefore, a pure aluminum material that exhibits excellent extrudability has been used as the tube material.
It is expected that heat exchangers will be further reduced in weight, and tubes will be further reduced in weight. In this case, it is necessary to increase the strength of the tube material. In recent years, CO2 (natural refrigerant) has been used instead of a fluorocarbon in order to prevent global warming. A CO2 refrigerant requires a high operating pressure as compared with a fluorocarbon refrigerant. This also makes it necessary to increase the strength of the tube material.
It is effective to add Si, Cu, Mn, Mg, etc. in order to increase the strength of the tube material. When the brazing target material contains Mg, a fluoride flux that is melted during heating reacts with Mg in the material to produce compounds such as MgF2 and KMgF3. This reduces the activity of the flux, so that brazability significantly deteriorates. The operating temperature of a heat exchanger using a CO2 refrigerant reaches as high as about 150° C. Therefore, intergranular corrosion susceptibility significantly increases when the material contains Cu. The refrigerant leaks at an early stage when intergranular corrosion has occurred, and impairs the function of the tube of the heat exchanger.
Therefore, Si and Mn must be added in order to increase the strength of the tube material. When adding Mn and Si to an alloy at a high concentration, Mn and Si dissolved in the matrix increase the deformation resistance of the alloy. For example, when a several hundred to several thousand extrusion ratio is employed (e.g., when producing a multi-port extruded tube), the alloy exhibits significantly inferior extrudability as compared with a pure Al material. An alloy that requires a high extrusion ram pressure or has a low critical extrusion rate (i.e., the maximum extrusion rate obtained without causing breakage of the partition wall of the hollow portion of the multi-port tube) exhibits inferior extrudability. An alloy containing Mn and Si at a high concentration requires a ram pressure higher than that of a pure Al material, so that the die tends to break or wear. Moreover, productivity decreases due to a decrease in critical extrusion rate.
For example, a method that adds Si and Mn that increase strength, and performs a high-temperature homogenization treatment and a low-temperature homogenization treatment in order to improve extrudability to reduce the amount of solute elements dissolved in the matrix and reduce the deformation resistance has been proposed. In this case, since an amount of solute elements is added, an improvement in extrudability (particularly an improvement in extrusion rate) is limited although an increase in strength may be achieved. Specifically, it is difficult to achieve a high strength and high extrudability (i.e., productivity) at the same time.
A refrigerant leaks from a refrigerant tube of an automotive heat exchanger when perforation corrosion has occurred during use. Therefore, Zn is caused to adhere to the surface of an extruded refrigerant tube by thermal spraying or the like, and is diffused by brazing. A Zn diffusion layer formed in the surface of the tube serves as a sacrificial anode for the deeper area, and suppresses corrosion in the thickness direction to increase the perforation life. In this case, a Zn application step (e.g., Zn thermal spraying) is required after extruding the tube. Moreover, a step of applying a fluoride flux required for brazing, or a step of applying a flux to the entire heat exchanger core must be performed after the Zn application step. This increases the production cost. Since the tube is not provided with a filler metal, it is necessary to use a brazing fin that is clad with a filler metal. This also increases cost as compared with the case of using a bare fin material that is not clad with a filler metal.
As a method that solves these problems, a method that applies a mixture of a filler metal powder and a Zn-containing flux powder to the surface of an aluminum alloy extruded refrigerant tube has been proposed. In this case, since the filler metal, Zn, and the flux can be simultaneously applied by a single step, the cost can be reduced. Moreover, since a bare fin material can be used, the cost can be further reduced. However, the above method does not necessarily provide the refrigerant tube with strength, extrudability, and corrosion resistance. A refrigerant tube that contains 0.5 to 1.0% of Si and 0.05 to 1.2% of Mn has also been proposed. In this case, a high strength may be achieved due to a large amount of solute elements, but an improvement in extrudability (particularly extrusion rate) is limited. Specifically, it is difficult to achieve a high strength and high extrudability (i.e., productivity) in combination.
JP-A-2005-256166, JP-A-2006-255755, JP-A-2006-334614, and JP-A-2004-330233 disclose related-art technologies.